[deb] Wrong architecture format in generated Release files

Run packetary:

$ packetary --threads-num 10 clone \
  -t deb \
  -r ubuntu_mirror.yaml \
  -R ubuntu_packages.yaml \
  -d .

ubuntu_mirror.yaml:
- name: ubuntu
  path: /home/alav/fuel/build/f_isoroot/f_repobuild/nailgun/mirrors/ubuntu
  priority: null
  section:
  - main
  - multiverse
  - restricted
  - universe
  suite: trusty
  type: deb
  uri: http://mirror.zetup.net/ubuntu/

ubuntu_packages.yaml:
mandatory: exact
packages:
- name: acpi-support
[...]

Everything goes well, but the output artifacts have one small format problem:

$ grep Architectures: nailgun/mirrors/ubuntu/dists/trusty/Release
Architectures: x86_64

The correct format for "Architectures" is the same as `dpkg-architecture -qDEB_HOST_ARCH1`, i.e. in this case "amd64" instead of "x86_64".

To me, the rootcause looks like a missing translation step between the two formats in [1] for "repository.architecture".
I'll attach a patch for this; I can also submit it via gerrit if it looks ok.

[1] https://github.com/openstack/packetary/blob/master/packetary/drivers/deb_driver.py#L412